1. Field of the Invention
The present invention relates generally to transmission of electronic signals between components of an electronic device, and more specifically to transmission of control signals between a controller and an optical pickup module in an optical data storage system.
2. Description of the Related Art
Optical data storage systems such as compact disc (CD) devices, Digital Video Disc (DVD) devices and Blu-ray Disc devices record to and read data from optical media. Figure (FIG. 1 is a schematic diagram illustrating components in a conventional optical data storage system 100. The example optical data storage system 100 of FIG. 1 includes a controller 110, an optical pickup unit 120, an actuator unit 130, and a motor assembly 144. The controller 110 performs various signal processing (e.g., error correction, encoding and decoding) and control functions for operating the optical data storage system 100. The controller 110 is operatively connected to the optical pickup unit 120 via an interface 114 and is operatively connected to the actuator unit 130 via a different interface 118. The interfaces 114, 118 are used for communication between the controller 110 and the connected components 120, 130 and may comprise multiple signal channels operating in parallel.
The optical pickup unit 120 includes a laser source (e.g., a laser diode) and a module for driving the laser source. The laser source is used to emit a laser beam 122 onto an optical disc 140 that is rotated by the motor assembly 144. A conventional optical pickup unit 120 has two different operating modes. In a write mode, the optical pickup unit 120 receives a control signal from the controller 110 over the interface 114 and emits the beam 122 with an intensity that is determined by the control signal. In the write mode, the intensity of the beam 122 is sufficient to form patterns on the tracks of the optical disc 140. The patterns represent data stored on the optical disc 140 and can later be retrieved in a read mode. In the read mode, the optical pickup unit 120 emits the laser beam 122 and uses a sensor to detect a reflected version of the beam. The reflected version of the beam represents data that has been retrieved from the optical disc 140. The optical pickup unit 120 sends the retrieved data to the controller 110 over the interface 114 for processing.
The actuator 130 moves the optical pickup unit 120 in a radial direction relative to the optical disc 140 according to control signals received from the controller 110 via the second interface 118. In this way, the optical pickup unit 120 can read or write data on different tracks of the optical disc 140.
FIG. 2 is a block diagram illustrating an example of a conventional optical pickup unit 120. The optical pickup unit 120 may include amplifiers 202A through 202C. Each of the amplifiers 202A through 202C is connected to a signal channel 114A through 114C in the interface 114 to amplify a control signal received over the interface 114. The amplified signals 203A through 203C are sent to current generators 210 to generate output currents 212A, 212B, 212C. The components of the control signal and the corresponding amplified signals are typically binary. In other words, each amplified signal has a value of zero (0) or one (1).
When an amplified signal 203 has a value of one (1), the corresponding current generator 210 is switched on. Each current generator 210 may generate a different level of output current 212 when switched on. For example, current generator 210A may generate an output current 212A of 20 milliamperes (mA), whereas current generator 210B may generate an output current 212B of 60 mA and current generator 210C may generate an output current 212C of 10 mA. When switched off, no current is generated by the current generators 210A through 210C. A current adder 214 adds the output currents 212 to generate a driver current 216 that is provided to a laser diode 218. The laser diode 218 outputs a laser beam 122 of differing intensity based on the level of the driver current 216. As a result, the magnitude of driver current 216 and the intensity of the laser beam 122 vary depending on which of the current generators 210A through 210C are turned on or off.
FIG. 3 is a timing diagram of a driver current 216, the corresponding laser power level, and a corresponding control signal in a conventional data write scheme. A power level of the laser beam 122 is labeled with a pair of letters (e.g., PR, PF, PC, etc.), and each laser power level is generated by sending a different level of driver current 216 into the laser diode 218. A data write scheme is a sequence of power levels that is used to form a pattern on a track of the optical disc 140, thus writing data to the optical disc 140. In the data write scheme of FIG. 3, the level of the driver current 216 is varied to generate the following sequence of laser power levels: PR, PF, PC, PF, PB and PR. Each driver current level 216 is defined by a combination of active channels 114A through 114C in the control signal. For example, only channel 114C is active for the driver current level 216 corresponding to PR whereas channels 114A through 114C are active for driver current level 216 for PF. In FIG. 3, the zeros (0) represent that a channel is inactive (i.e., turned off) whereas the ones (1) represent that a channel is active (i.e., turned on). An active channel causes the corresponding current generator 210 to be switched on, thus generating the driver current 216.
As illustrated in FIG. 3, the control signal transmitted over the interface 114 undergoes a series of transitions 300 (e.g., active to inactive state and inactive to active state) to change the driver current level 216. A signal sent over the channels 114A through 114C may become skewed or distorted due to electronic noise, crosstalk, or electromagnetic interference (EMI). In particular, when one channel of the signal undergoes a transition 300 from zero to one or vice versa, the channel and the driver current 216 may take some length of time to settle, and this settling time may increase as the level of skew or distortion increases. In addition, the settling time for the driver current 216 may be longer when multiple channels of the signal undergo a transition simultaneously. For example, the transition 300C between the PC and PF current levels (e.g., when only one channel undergoes a transition) would have a shorter settling time than the transition 300D between the PF and PB current levels (e.g., when all three channels undergo a transition). To safeguard against errors that may occur during the longer settling times associated with simultaneous channel transitions, such as the transition 300D, the data transmission rate is decreased. This results in overall transmission inefficiency.